Electrostatic cleaner and method



Oct; 27, 1970 w. F, )5 G E -r 3,53528 ELECTROSTATIC CLEANER AND METHOD Filed Aug. 16, 1968 mm n" 1 I m" m" m 1 CU l l l l l H HULK" a 3 8 NV M P? *4 .2 9 0 ON 2 E m a 4! i: Q R .i

United States Patent 3,536,528 ELECTROSTATIC CLEANER AND METHOD Wilfried Florent De Geest, Berchem, Belgium, assignor to Gevaert-Agfa N.V., Mortsel, Belgium, a company of Belgium Filed Aug. 16, 1968, Ser. No. 753,245 Claims priority, application Great Britain, Aug. 16, 1967, 37,762/ 67 Int. Cl. B08b 7/04 US. Cl. 134-1 14 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for cleaning particles from the surface of a web by exposing one face of the web to a corona discharge, passing the web over an electrode of like charge on the opposite side of the web, and removing the particles with air currents.

The present invention relates to the electrostatic removal of fine particles from sheet material, particularly flexible sheet material in web or strip form which can be conveyed through a cleaning station either directly by rollers or while supported on an endless conveyor belt. By way of example the invention is applicable to the cleaning of coated and uncoated paper, plastics and woven textiles.

Any fine particles can be removed by means of the invention, e.g., organic or inorganic dust, fibre or other particles, whether they are on the surface of the sheet material, or, in the case of woven materials, in the interstices thereof.

As the invention has been developed primarily for use in the manufacture of light-sensitive and other materials used in photographic processes, reference is particularly made to such materials in this specification but it is to be understood that the invention has many other poten tial uses.

In the manufacture of photographic film, it is ex tremely important to clean the transparent film web as thoroughly as possible before subbing, light-sensitive or other layers are applied. If, for instance, a film of the negative type is produced, i.e., a film wherein a negative image of the original will be formed, the presence of fine particles in the layer(s) of the film will cause black spots upon projection, and consequently white spots upon projection of the positive copy therefrom.

Alternatively, when the fine particles adhere to the negative film during the exposure thereof, but are removed after processing, the positive copy of said film 'will show a plurality of black spots.

Thus, it is clear that in the manufacture and the treatment of photographic material, all kinds of fine particles should carefully be removed from the webs to be treated.

Known apparatus for the removal of dust do not work very satisfactorily when used for cleaning photographic material, because, if they work merely electrostatically, their elfect is not always sufiicient, and if they apply also a mechanical treatment to the web such as brushing, beating, or the like, they may intolerably damage the surface of the photographic material.

According to the present invention, particles are removed from sheet material by applying positive or negative electric charges to one side of the material by corona discharge, subsequently exposing the other side of the material to one or more backing electrodes bearing electrostatic charges of the same polarity as the charges deposited by the corona discharge and of such magnitude as to repel particles from said one side of. the material 7 by the electrostatlc forces and removing such repelled particles from the vicinity of the material.

The invention also includes apparatus for performing the said cleaning method. In particular the invention includes apparatus comprising an electrostatic cleaning station and means for conveying a strip or web of material through such station, which latter comprises at least one corona discharge device disposed for depositing charges of a given polarity on one side of a strip or web during its conveyance through the station, one or more backing electrodes disposed downstream of said corona discharge device(s), on the other side of the web path, and connected to a DC. source by which the said electrode or electrodes may be maintained electrically charged at high potential of the same polarity as the charges deposited by said corona discharge device or devices, and means for removing from the cleaning station particles repelled from the material by the electrostatic forces.

The removal of freed particles from the vicinity of the material is preferably achieved by one or more air currents maintained by suction and/or blowing devices. Such air current or currents is or are preferably set up along or across the surface of the material.

Although the particles are mainly freed at the side of the web which received first a charge of a given polarity, the particles at the opposite side of the web are also freed to a given extent by the electrostatic field established by the backing electrode(s) at that side, and in case it is desired to clean also said opposite web side, an air current may be maintained at said opposite side.

In case it is desired to thoroughly clean the web at both sides, and the cleaning at the mentioned opposite side as described hereinbefore is not sufiicient, the cleaning operation can be repeated, the web sides being reversed.

In the case of apparatus for cleaning a moving web or strip (hereafter called web) the corona discharge device(s) and the mentioned backing electrode(s) preferably extend substantially transversely of the web.

Means is preferably provided for electrically neutralising the sheet material after particles have been removed from one or each side thereof.

By way of example, a DC. potential of about 7,000 volts may be applied to the corona discharge device(s), and a DC. potential of 100,000 volts may be applied to the backing electrode(s).

The invention will hereinafter be described by way of example in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic view illustrating the principles of the cleaner according to the invention;

FIG. 2 is a diagrammatic view of a cleaner installation.

In the arrangement according to FIG. 1, a web 10 is conveyed over conveyor rollers 11 and 12 which are electrically grounded, and is driven in the direction of the arrow by means, not shown.

A corona discharge device 13 extends transversely of the path of the web and comprises a straight wire which runs parallel to the plane of the web at a distance of about 5 mm. therefrom. The wire is connected to a source 14 of DC. tension, to obtain in respect of the ground a negative potential. A shield 15 surrounding the wire is grounded.

About 10 cm. downstream of the device 13 a backing electrode 16 is located at the backside of the web. It consists of a rectangular copper plate with rounded off corners which is mounted electrically insulating and parallel to the web at about 0.2 cm. distance thereof. The electrode is connected to a Van de Graaff high tension generator 17, one terminal of which is grounded Further 10 cm. downstream a neutralizing station is 0 provided, comprising two corona discharge devices 19 and former 21, the centre of which is grounded. The primary is connected to a source 22 of AC.

The operation of the arrangement is illustrated by the following example.

A web of polyethylene terephthalate having a thickness of 100 am. and a width of cm. was transported over the rollers -11 and 12 at a speed of 20 cm./ sec.

The surface of the web carries particles of talcum sized between 0.1 and 0.01 ,uJIL, web fragments sized between 0.1 ,uIll. and 2 mm., and particles of metallic kind, e.g. copper, ranging around 1 m.

The potential of source 14 amounted to 7000 volts, that of source 17 amounted to 150,000\ volts, and the transformer 21 was connected to the main supply, the total potential of the secondary amounting to 7000 volts, 50 cycles.

The electrode 16 measured 60 by 20 mm., the longer side extending transversely of the web.

In the operation of the arrangement, the application of a uniform layer of negative charges to the web and to the particles lying thereon by wire 13 caused a first, although slight, decrease of the adherence of the particles to the web. This was noticed when the removal of the particles by an air stream impinging on the web as indicated by the arrows 23 was compared to the removal of the particles when the source 14 was switched off.

The almost complete reduction of the adherence of the particles to the web occurred, however, over the backing electrode 16. As a consequence of the electric field established by the electrode 16 having the same polarity as that of the charges applied by the device 13, very strong repelling forces were created which caused the particles to literally jump on the web surface when no air stream was removing them. The establishment of the air stream indicated by the arrows 23, led to the complete removal of the freed particles.

The discharging of the web by corona devices 19 and 20 restores the electrically neutral condition of the web so that there is no risk of dust and other particles being attracted from the atmosphere onto the web immediaely after its cleaning.

In case it is desired to clean both sides of the web, it may be passed through a second cleaner which acts on the non cleaned side of the web. In the case of particles which do not strongly adhere to the web, a sufficient cleaning may be obtained at the opposite side of the web by establishing at said opposite side also an air stream which removes the particles freed by electrostatic field established by the backing electrode 16.

The arrangement just described shows the principles on which the invention is based. For practical operation, the cleaner will generally show some additional features which will be described hereinafter in connection with FIG. 2.

A web 30, the driving means for which is not shown, enters a housing 31 through opening 32, and is passed over an idler roller 33 to a backing roller 34. The Web contacts the backing roller over about 90 angular degrees and is further passed over an idler roller 35 to leave the housing through slotlike opening 36.

Between rollers 33 and 34 a corona discharge device 37 is provided which has the same function as the device 13 of FIG. 1. Between rollers 34 and 35 a discharge station 38 is provided, similarly to neutralizing device 19 of FIG. 1.

The housing 31 air-tightly surrounds the parts just described, except for the entrance and outlet openings 32 and 36. Through a rectangular opening at the top side a head 40 projects downwardly.

The head 40 has a concave underside which closely surrounds, say at a distance of 1 mm. or so, the Web conveyed over the roller 34. The head is provided with two wedge-like channels 41 and 42 which have a rectangular cross-section. Through a duct 43 pressurized air is fed to the channel 41 whereas through a duct 44' the air laden with particles removed from the web is sucked off.'The

width of the opening of the channel 41 facing the web is relatively small so that a knife-like stream of air is directed onto the film, Whereas the width of the opening of channel 42 is relatively great compared to that of channel 41.

The backing roller 34 which is electrically conductive has an insulating peripheral layer 45 of PVC, Teflon, or the like, of a thickness of about 1 mm. The roller is journalled in insulating bearings, and is electrically connected to a high tension D.C. source.

The use of a roller as backing electrode olfers some interesting aspects.

First, the conveyance of the web over a roller positively determines its course. In this respect it may occur that difficulties arise with the arrangement shown in FIG. 1, wherein the web part located between rollers 11 and 12 may start to vibrate, thereby possibly touching the electrode 16, or may tend to be repelled from the electrode 16 as a consequence of the charges onto the web having the same electrical sign as the polarity of said electrode. In both cases the uniformity of web travel is impaired and difficulties may be encountered in ulterior treatments of the web, e.g. when the web is to be coated immediately after leaving the cleaner.

Second, the curving of the web over the roller creates a slight tension in the convexly curved side of the web whereby the adherence of particles to said side is slightly decreased, and their removal is facilitated.

The air stream leaving the opening of channel 41 strikes in a laminar stream over the web surface and becomes laden with particles, on the one hand because the particles have been freed from the Web by the phenomenon of electrical repellence, on the other hand as a consequence of the turbulence created at the area where the air stream impinges obliquely onto the web surface.

The underpressure prevailing in channel 42 does not substantially contribute to the loosening of particles from the web, but is mainly necessary for carrying oif the quantum of air introduced into the apparatus through channel 41.

The electrostatic cleaner according to the present invention is not limited to the apparatus described hereinbefore.

The backing roller may be provided at the lateral sides with projecting rim portions which fit in corresponding grooves in the concave side of the head 40. Thereby the lateral air escapes through the annular opening between roller 34 and the head 40 may be kept very low, so that the housing 31 may be omitted.

Details for designing the cleaning head 40 and cooperating roller 34, are contained in the description of the Belgian patent specification 6903006 filed Nov. 22, 1966 by Gevaert-Agfa N.V., dealing with an apparatus for the contactless removing of dust from webs and that specification is deemed to form part of the present disclosure.

The invention is not limited to the embodiments described so far.

The backing electrode need not necessarily be a plate or a roller as described, but may be constituted by a flat or curved metal strip a piece of conductive fabric or mesh, or one or more wires or threads extending transversely of the Web and which even may take the form of the corona device shown at 13 or 37.

The corona devices may comprise one or more tensioned wires within a screening as shown in the figures, but they may comprise also a transversely extending elongate metal bar or bath, the smaller side facing the web side having preferably a width not greater than 2 mm.

The reduction of the adherence of the particles to the web by the charges deposited by the corona devices 13 and 37 may be improved in making the web follow at said locus a convexly curved path. In the arrangement according to FIG. 1 for instance, this may be done in moving the corona device 13 more to the left so that it becomes is curved around the roller.

In case the backing electrode has the form of a plate or a roller as shown, which faces the web over at least some centimeters, the DC potential applied thereto shall amount to at least 30,000 volts. This voltage need not necessarily be supplied by a Van de Graatf generator as shown, but may be any electric or electronic power supply which is capable of delivering a voltage of the mentioned magnitude.

In connection with the magnitude of the DC. voltage, it is pointed out that the DC. voltage for feeding the corona devices need not have a smooth profile, but that it may also be, and in cases wherein a particularly high voltage is required even preferably is, a pulsating D.C. tension such as the one produced by means of an oscillating power electron tube and a pulse transformer. A preferred waveform of a pulsating DC. voltage shows a steep pulse which passes into an almost horizontal part the magnitude of which is much smaller than the pulse peak. In a pulsating DC. voltage suited for use in the present invention, the pulse peak may amount to 500,000 volts, the value of the flat portion of the period may be 5000 volts, and the frequency may be about 50 kilocycles. The advantage of the described waveform is that the peak ensures the effective ionisation of the air, whereas the flat wave portion suffices for the maintenance thereof without causing'any damage to the Web.

The source 22 feeding the device for electrically neutralizing the web may be also a HP. generator. The frequency of which is sufiiciently high to leave no detectable electrical ripple pattern on the web after its neutralization.

In fact, in the case of relatively high web speeds say greater than 1 m./sec., a relatively low frequency of the source 22, say 1 kc., and coating high-sensitive or extremely thin light-sensitive emulsion layers immediately after the web has been cleaned in the way described, it may occur that the period of the AC. tension feeding the neutralizing station, or in other words, the electrical ripple pattern resulting therefrom onto the web, may become visible after the exposure and processing of the web, as a consequence of a non-uniform coating thickness.

What I claim is:

1. A process wherein particles are removed from sheet material by applying positive or negative electric charges to one side of the material by corona discharge, subsequently exposing the other side of the material to one or more electrodes bearing electrostatic charges of the same polarity as the charges deposited by the corona discharge and of such magnitude as to repel particles from said one side of the material by the electrostatic forces, and removing such repelled particles from the vicinity of the material.

2. A process according to claim 1, comprising electrically neutralizing the sheet material after the removal of the particles therefrom.

3. Apparatus for use in removing fine particles from a web, comprising an electrostatic cleaning station and means for conveying a strip or web of material through such station, which cleaning station comprises at least one corona discharge device disposed for depositing charges of a given polarity on one side of a strip or web during its conveyance through the station, at least one backing electrode disposed downstream of each such corona discharge device, on the other side of the web path, and connected to a DC. source by which the said electrode or electrodes may be maintained electrically charged at high potential of the same polarity as the charges deposited by said corona discharge device or devices, and means for removing from the cleaning station particles repelled from the material by the electrostatic forces.

4. Apparatus according to claim 3, wherein said at least one corona discharge device and each such backing electrode extend transversely of the path followed by the web.

5. Apparatus according to claim 3, wherein a DC. potential of at least 30,000 volts is applied to said electrode.

6. Apparatus according to claim 3, wherein said backing electrode is in the form of a corona discharge device.

7. Apparatus according to claim 3, wherein each of said corona discharge devices comprises one or more elongate electrodes, the side of which facing the web has preferably a width not greater than 1 mm.

8. Apparatus according to claim 3, wherein the clean ing station is located in a chamber through which the web is passed, and which is provided with means for directing air in the chamber obliquely towards the film.

9. Apparatus according to claim 3, wherein said backing electrode is in the form of a roller and wherein said web is passed over an arc of the periphery of said roller 10. Apparatus according to claim 9, wherein said roller has an electrically insulating peripheral layer.

11. Apparatus according to claim 3, wherein the electrostatic devices located at both sides of the web path are connected to a source of DO. tension which produces a DC. voltage of at least 5,000 volts.

12. Apparatus according to claim 11, wherein the voltage produced by the source of DC. voltage is a pulsating DC. voltage, one period comprising a steep pulse the peak value of which is at least 50,000 volts and a flat wave portion the voltage of which is at least 5,000 volts.

'13. Apparatus according to claim 3, wherein means is provided for electrically neutralizing the web after the particles have been removed therefrom.

14. Apparatus according to claim 13, wherein said means for neutralizing comprises a corona discharge device located at least at said one side of the Web and which is connected to a source of A.C.

References Cited UNITED STATES PATENTS 2,752,271 6/19'56 Walkup et al 1341 2,920,987 l/1960 Landry et a1 151.5 XR 2,956,301 10/ 1960 Bruno.

2,965,756 12/1960 Vyverberg 134-1 XR 2,980,933 4/1961 Schwartz et al 15-1.5 3,395,042 7/1968 Herbert 134-1 JOSEPH SCOVRONEK, Primary Examiner D. G. MILLMAN, Assistant Examiner US. Cl. X.R. l5-1.5; 3l7262 

